The present embodiments relate to single photon emission computed tomography (SPECT). In particular, the effects of scatter are reduced in SPECT.
In SPECT, a radioactive substance is administered to a patient. An imaging detector detects, through a collimator, some of the γ-radiation emitted from the patient. The detected emissions are tomographically reconstructed to generate an image object of locations of the emissions in a patient. Due to the gamma radiation scattering, some of the detections are of scatter rather than direct or primary photon emissions. This scattering degrades the SPECT image, so is to be reduced. The collimator typically has parallel holes to filter out photons that are not from particular directions. The scattering process is typically Compton scattering, which relates a change in direction with a reduction of energy as described by the Compton scattering formula. Scattered photons can fulfill the directionality condition as imposed the specific collimator, and it may even fulfill the energy acceptance conditions. Typically the energy window must be set large enough to allow for all primary photons to enter, however a large fraction of all detected counts are scattered photons. From simulations, the scatter fraction in the photo peak may be as high as 50% or more.
Scatter may be a problem for collimators with non-parallel holes (non-parallel-hole collimators). Non-parallel-hole collimators are usually used to achieve better tradeoff between sensitivity and resolution for imaging certain organs (e.g., heart, thyroid and brain). The non-parallel-hole collimators greatly increase the difficulty in accurate modeling of the image formation process, including scatter. Thus, the quantitative accuracy of SPECT imaging using non-parallel-hole collimators is usually poor.
To improve the image formation process, energy-window based scatter correction has been used for non-parallel-hole collimators. The scatter at energy windows adjacent to but not at an energy window for the primary photons is measured. The scatter at the energy window for the primary photons is estimated by linear interpolation from the scatter in the adjacent windows. This estimate is used in image formation to reduce the contribution of scatter. This-energy window type of scatter correction may be inaccurate or rely on assumption in interpolation that are not true.